Many little parasites add up to one big biomass

Parasites are small, but they punch above their weight in terms of their effects on other life forms. Now it turns out that the amount of parasites in an ecosystem physically weighs more than the top predators.

It was previously thought parasites did not contribute much biomass when put against that of other animals and plants. To check this, Armand Kuris of the University of California, Santa Barbara, and colleagues painstakingly estimated the biomass of animals, plants and parasites in three estuaries in California and Baja California.

They sampled 23 random sites in each estuary and estimated the biomass of all life-forms in each of those sites, from tiny parasites up to birds – the local top predators.

‘Castration’ effect

Put together, the parasites in each estuary weighed as much as the local fish and between 3 and 9 times more than the local birds. Parasites that “castrate” their hosts – prevent them from reproducing – had the greatest cumulative biomass and on their own weighed as much as the local winter birds.

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For Andrew Dobson of Princeton University, one of Kuris’s collaborators, this shows that that castration is the most effective way for parasites to survive. Castration allows parasites to keep the hosts alive while preventing them from spending any energy on their own reproduction.

“All free-living organisms host one or more parasites,” says Peter Olson of the Natural History Museum in London. For this reason, it has long been thought that parasites are likely to be the most common life-form on Earth in terms of numbers of species, and possibly numbers of individuals.

“However, in terms of biomass, parasitologists and non-parasitologists alike have generally assumed parasite biomass to be negligible in comparison with free-living organisms,” Olsen says. “Kuris and colleagues’ finding is surprising.”

Dobson says he has work in press showing that parasites contain between 10 and 1000 times more pollutants than their hosts. He suggests parasites might play an important role in cleaning up the animals they inhabit.